In the production of terephthalic acid, slurry streams containing terephthalic acid crystals may arise at one or more stages in the process and the nature of the liquor in which the crystals are slurried may vary. For instance, where the crude terephthalic acid is initially produced by the liquid phase oxidation of paraxylene in the presence of a carboxylic acid, such as acetic acid, and a suitable catalyst system (typically cobalt, manganese and bromine compounds), the crude terephthalic acid is withdrawn from the reactor as a slurry of terephthalic acid crystals in liquor comprising acetic acid and water with dissolved impurities, including terephthalic acid precursors such as 4-carboxybenzaldehyde (4-CBA) and paratoluic acid. If the crude terephthalic acid is thereafter purified by hydrogenation of an aqueous solution thereof (possibly preceded by a further stage of oxidation of the crude terephthalic acid in aqueous solution to convert 4-CBA to terephthalic acid), a slurry of purified terephthalic acid in aqueous liquor results where the aqueous liquor may have dissolved therein impurities such as paratoluic acid. In both cases, the terephthalic acid, either crude or purified, has to be freed of the corresponding liquor in a highly efficient manner.
A suitable technique in this latter respect is afforded by a combined filtration and washing system such as that disclosed in our prior published EP-A-502628 and copending International patent application No. PCT/GB 93/01019, the entire disclosures of which are incorporated herein by reference. In the systems disclosed in these prior applications, the displacement of liquor from wet filter cake comprising terephthalic acid (crude or after purification) is effected by transporting the filter cake on a belt filter through a washing zone in which an aqueous wash liquid is supplied to the filter cake in a number of stages at different points along the path of travel of the belt. The wash liquid displaces the liquor from the filter cake and the liquor together with the wash liquid passes through the filter material forming the belt.
In practice, it is necessary to establish an atmosphere of inert gas over the filter cake, for instance to exclude or control the level of oxygen present and/or to assist in drying of the filter cake. This inert gas may pass through the filter material in the washing zone and/or in a zone or zones downstream of the washing zone. Additional gas must be introduced in order to maintain the gaseous atmosphere. The inert gas may be nitrogen for instance, although in the case of the filter/washing system employed in the purification stage of the process, the inert gas may with advantage comprise steam for reasons disclosed in our co-pending International patent application No. PCT/GB 93/01033 (the entire contents of which are incorporated herein by reference).